River confluences are critical points in river networks where strong physical and
chemical gradients develop, resulting in a wide range of distinctive environmental
conditions (habitats) for biological growth. Large variations in water temperatures,
organic matter, nutrients, for example, and in general, in water chemistry have been
reported to occur at these sites. As a consequence of their high spatial and temporal
heterogeneity of habitats and resources, river confluences behave as biological hotspots,
where the number of species appears to increase very significantly in comparison with
other river reaches. The effects of river confluences persist downstream, therefore,
affecting biological communities and ecological processes at scales of river reaches and
channel networks. The spatial extent of the reaches downstream of river junctions where
heterogeneous habitat conditions persist, largely depends on the rate at which mixing
between the mainstream and tributary waters occurs. The literature on mixing in river
confluences is extensive, but still, our understanding of flow and mixing dynamics in
these sites is far from complete. In particular, the effect of density contrast between the
confluent streams on mixing has traditionally been neglected, which has been justified by
differences in the inertia of the confluent flows being much higher than density
differences. However, as the scale of the confluent channels increases, the probability of
draining different geological terrains also increases which results in an increasing
potential for significant differences in density.
In this work, we present results of a series of field experiments carried out in a
confluence in Northern Spain where the presence of density contrast is important for both
the spatial arrangement of the rivers once at the confluence and river mixing.

Sponsorship:

Tesis Univ. Granada. Programa Oficial de Doctorado en: Ciencias de la TierraWork on the confluence between the
Ebro and Segre rivers was funded through a collaborative agreement between the
University of Barcelona and the University of Granada to work jointly in the project
“Gestión hidráulica y técnicas de detección remota aplicada al control de poblaciones
mejillón cebra: el caso del embalse de Ribarroja y el tramo inferior del río Ebro”, funded
by the Spanish Ministry of the Environment. Work on the Sacramento River was funded
through the project “North Delta Salmon Out-migration Study”, funded by the United
States Department of Interior ― Bureau of Reclamation (USBR), solicitation No.
09SS200013. The PhD student was supported by a PhD grant (Programa Estatal de
Promoción del Talento y su Empleabilidad, subprograma de Formación de Profesorado
Universitario) from the Spanish Government.

Publisher:

Universidad de Granada

Keywords:

HidrologíaRíosHidrodinámicaDinámica de fluidosAnálisis por sedimentaciónEcología de ríosFauna de ríos